1. Field of the Invention
The present invention relates generally to VTOL vehicles, and more particularly to an improved VTOL vehicle having ducted fan propulsion systems wherein nacelles forming ducts housing the engines can be tilted, and the thrust can be selectively vectored by tilting the nacelles and actuating adjustable vanes mounted in the aft portion of each nacelle duct.
2. Brief Description of the Prior Art
Several attempts have been made to create a vehicle that can be flown in the air as well as driven on land. It is desirable that such craft be capable of vertical takeoff/landing (VTOL) in order to minimize the runway space necessary for lift-off. One approach to achieve required lift for take-off/landing and thrust for conventional level flight includes the tilting of engine driven fans or propellers to vector the thrust from the vertical to the horizontal direction. Another approach mounts fans in a fixed position and various means providing for deflecting the flow generated by the fan to achieve the desired thrust vectoring.
The first approach can be found in U.S. Pat. No. 5,839,691 to Lariviere. Therein, the disclosed VTOL aircraft has propellers the rotational axis of which is rotated through 90 degrees from the vertical to the horizontal direction as the aircraft transitions from lift-off to its cruising configuration. Such a tilt-rotor configuration induces the mass flow of air displaced by the propeller around the aircraft""s lifting surfaces, known as propeller wash. The propeller wash is significant at the early stage of transition from vertical to horizontal flight and the final stage of transition from horizontal to vertical flight, as the rotational axis of the propeller makes a large angle with respect of the horizontal direction. The propeller wash affects the aerodynamic forces and moments of the aircraft causing profound difficulties in aircraft control. In addition, at the transitional stages, the wing can generate negative lift due to the propeller wash and the engine must produce extra thrust to compensate for the negative lift. Also, a large aerodynamic drag is generated on the wings during hovering and reduces payload. To handle the effects of propeller wash, extra structure for supporting the propellers and additional control surfaces are required.
This approach is user unfriendly with large rotating open blades. In addition, in a case like the V22 Osprey, the aircraft cannot land conventionally with the rotors transitioned to the forward flight position because the rotors are so large that they would contact the ground. Another disadvantage of this approach is that the rotors of the aircraft must be geared together by a gearbox to prevent catastrophic failure should an engine fail, and gearbox failure is the major mechanical failure mode in the aircraft.
The alternative to the first approach is to use ducted fans, which rotate through 90 degrees. This approach is more user friendly and the aircraft can land conventionally on a standard runway should the engine fail. However, ducts that rotate through the full 90 degrees can create leading edge stall at some point in the transition where the duct is still at a high angle and the aircraft is flying quite fast. If this airflow stall occurs, a successful transition is unlikely. Many VTOL aircraft employing this approach have crashed. Applicant""s present invention overcomes this ducted fan problem by only partially rotating the ducts and then using deflection vanes to complete the redirection of the thrust. Thus, the ducted fans rotate to the horizontal position (fan centerline in direction of flight) very early in transition when the horizontal velocity is still low. This eliminates duct leading edge separation. Thrust for transitioning from hover to forward flight in accordance with the present invention occurs by creating a small reduction in the angle of orientation of the duct centerline relative to the angle of orientation of the fuselage centerline, which generates a substantial horizontal thrust.
The second approach is found in U.S. Pat. No. 4,358,074 to Schoen, wherein a propulsion system for VTOL aircraft has stationary ducts which vector the airflow by utilizing a movable, fixed camber, cascading vane system in addition to a slotted flap system. The airflow within each nacelle is divided into two streams. One of the streams is directed downwardly through the fixed camber vane system. The other stream is exhausted through an aft nozzle at the outlet, against a slotted flap system mounted on a wing located immediately behind the duct. The fixed camber vane system can only direct a limited amount of airflow through very modest angles before the flow separates creating thrust loss. The slotted flap system is required to deflect the full stream through large angles by utilizing the wing flap system alone.
Another flow deflection system that includes adjustable vanes is disclosed in U.S. Pat. No. 5,115,996 to the present inventor Paul Moller. In the Moller ""996 patent, a VTOL aircraft is disclosed having ducted fan propulsion systems wherein ducted nacelles housing the engines remain stationary with their axial center lines approximately parallel with the center line of the fuselage, and the thrust is selectively vectored by adjustable vanes mounted in the aft portion of each nacelle duct.
The inventor has found a disadvantage in this design, however. In this design, the adjustable vanes were required to bend the flow through 90 degrees. The problem with doing this is that the cross-sectional area of the flow as it moves through the vanes changes dramatically, as shown in FIG. 1A of this application. The air is assumed to flow in the passage from left to right, with the passage having varying cross-sectional dimensions. It can be seen that the cross-sectional diameter at point A where the air stream enters the passage is smaller than at point B, as it turns the corner, and then narrows again at point C where it exits the passage. As the flow approaches the turning section, the flow slows down until it passes the point B. At the same time, the pressure increases to generate an adverse pressure gradient inducing the rapid growth of a boundary layer. The boundary layer is the thin layer of flow near the surface and, in the current configuration, develops on the convex side of the vanes and the inner walls holding the lateral ends of the vanes. Upon further turning of the flow stream, the boundary layer thickness can grow to a considerable size, which can lead to flow separation and, as a consequence, reduce the effective cross-sectional area of the airflow and lower the efficiency of the vanes. One solution to this flow separation is to thicken the cross section of the vanes as shown in FIG. 1B to maintain constant cross sectional area of the flow at points A, B, and C as shown. This will help keep the flow velocity constant but will create an interference with the flow when the vanes are retracted as shown in FIG. 1C, as the distance at points A, B and C now will vary. In this case, the flow speeds up and slows down for no useful purpose with an associated pressure thrust loss. Another drawback of this solution is the development of a boundary layer near the trailing edge of the vanes, which will reduce the efficiency of the vane system.
As mentioned above, various aerodynamic and control problems of the first approach occur at the transitional stages. Also, the existing tilt-rotors have inherent complexities in the design of propellers themselves and structural mechanism for propeller cyclic pitch control. In the second approach, the efficiency of the vane systems declines as the flow turning angle increases. Thus, there is a need for an improved VTOL vehicle design having tiltable fans that cooperate with the adjustable vane systems to produce effective power modulation and thrust vectoring, operate in the ranges where the problems associated with an individual approach are not present, and do not contain the inherent complex structures related to propellers.
It is therefore a primary objective of the present invention to provide an improved VTOL vehicle that utilizes nacelles in conjunction with adjustable vane systems to eliminate duct leading edge stall during the transition of the flight mode, to enhance thrust efficiency, to eliminate the need for variable pitch fans and to reduce the tendency to suck foreign objects into the fans.
Another object of the present invention is to provide an improved VTOL vehicle that has mechanisms for tilting the nacelles and sensors for monitoring the tilting angle.
Yet another object of the present invention is to provide an improved VTOL vehicle that includes an adjustable vane system with enhanced thrust efficiency and reduced fluid mechanical losses.
Still another object of the present invention is to provide an improved nacelle configuration and orientation system that, in addition to generating prime mover thrust, can also be used to control vehicle pitch and roll.
A further object of the present invention is to provide an improved VTOL vehicle that may include either two or four nacelle mounted power plants.
An additional object of the present invention is to provide an improved VTOL vehicle having engine control systems with improved response characteristics for vehicle attitude control.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following detailed description of the preferred embodiments which make reference to several figures of the drawing.
Briefly, a preferred embodiment of the present invention includes a fuselage with two foldable wings, two tiltable nacelles attached to the wings, a vertical stabilizer, a horizontal stabilizer, and two auxiliary thrusters. Each nacelle contains a system of vanes located at the rear opening thereof, and means are provided for extending and retracting the vanes in conjunction with nacelle tilting mechanisms to deflect the airflow over a predetermined range of angles from the horizontal. Each nacelle also contains two rotary engines, each of which directly drives a fan. The fans face each other and operate in counter-rotating directions at the same rotational speed. An alternative embodiment includes two additional tiltable nacelles attached to the fuselage instead of having the auxiliary thrusters. A redundant computerized flight control system maintains stability of the vehicle as it transitions from one flight mode to another.